It is well known that hydrocarbon sulfonates such as petroleum sulfonates are useful in a wide variety of applications, particularly in the preparation of compounds and compositions which are useful as additives in lubricants and normally liquid fuels. Procedures for preparing such sulfonates have been described extensively in the prior art. The economically feasible methods of preparing metal salts of organic sulfonic acids include the steps of reacting organic compounds with a sulfonating agent, separating undesired components (acid sludge) from the sulfonated product, and thereafter treating the sulfonated product with a metal base to form metal salts. The sulfonation may be carried out with a variety of sulfonating agents such as sulfuric acid, chlorosulfonic acid, oleum, or sulfur trioxide. In spite of the removal of the acid sludge after sulfonation, the sulfonated product mixture resulting from the sulfonation of the organic compound still contains appreciable amounts of unchanged inorganic sulfonating agent and organic contaminants such as low equivalent weight organic sulfonic acid components.
The removal of the inorganic sulfonating agents from the sulfonated acid masses has been the subject of much study and often has involved expensive and inconvenient operating procedures. One of the difficulties resides in the mutual solubility characteristics of the sulfonated product and its salts with the inorganic acid contaminant and their salts. Another difficulty arises from the ability of the metal salts, especially the polyvalent metal salts, of the sulfonated products to act as dispersants for the metal salts of the inorganic acid contaminants and maintain the latter in suspension. The result often is a hazy solution which is unsuitable for many purposes.
The extraction of the inorganic contaminants (e.g., unreacted sulfonating agent) with aqueous solutions is not a complete solution to the problem since some of the water containing sulfuric acid remains dispersed in the product even after such extractions in the form of an aqueous emulsion. The sulfuric acid content will vary depending on the various processes utilized, but may be as high as 10-15% of the total weight of sulfonic acids and sulfuric acid.
The presence of sulfuric acid in the organic acid mass, even at low concentrations, is detrimental for many of the intended uses of the organic sulfonic acids. For example, when these organic sulfonic acids are neutralized with an alkali to prepare emulsifying compositions or rust-preventing compounds, the presence of an alkali metal sulfate may impair the emulsifying power of the organic sulfonate and even promote corrosion. When said acids containing minor amounts of sulfuric acid are neutralized with alkaline earth metal bases in order to prepare detergent additive oils, the impurity forms highly dispersed alkaline earth metal sulfates which makes the filtration of these additives difficult and decreases their dispersant power.
Several processes have been suggested in the prior art for reducing the inorganic sulfonating agent components in the sulfonated products, but many of these processes have not satisfactorily solved the problems. Processes have been suggested based on water extraction using oxygenated solvents, several successive extractions and at least one evaporation of the solvent. U.S. Pat. No. 4,087,456 describes a process for the production of hydrocarbon sulfonates involving the removal of undesired inorganic sulfites and sulfates and undesired hydrophylic hydrocarbon sulfonates utilizing combinations of inorganic hydroxides, water and neutral oleophilic organic compounds. Examples of the useful oleophilic organic compounds include alcohols such as pentanols, hexanols and octanols; alkyl phenols and cresols; aldehydes such as heptaldehyde; ketones such as cyclohexanone; esters such as butylbutyrate; and mixtures thereof. More specifically, the process involves adding to the sulfonated product mixture after neutralization with an inorganic hydroxide, from about 0.05 to about 10 moles of a neutral oleophilic organic compound and from 5 to about 500 moles of water, per mole of sulfonate groups in said product mixtures. U.S. Pat. No. 2,453,690 describes a process of producing polyvalent metal hydrocarbon sulfonates wherein water-soluble sulfonates ordinarily present in such mixtures are eliminated by diluting the batch of sulfonates undergoing metathesis or purification with a small amount, e.g., about 0.2 to 10% of a water-insoluble oil-soluble aliphatic alcohol or other oxygenated bearing organic solvent. Examples of such alcohols disclosed include amyl alcohol and butyl alcohol. Such alcohols are reported to be useful as emulsion-breaking liquids. Examples of other patents which describe the use of various oxygenated solvents in facilitating the purification of hydrocarbon sulfonic acid mixtures include U.S. Pat. Nos. 2,453,690; 2,924,617; 1,930,488; 2,168,315; 2,848,415; 3,242,080; 4,119,661; 4,177,208; and 2,739,982.
Processes for purifying organic sulfonic acid mixtures based on the precipitation of insoluble salts generally require longer reaction times and may give rise to difficulties relating to the separation of the precipitate which can be extremely fine. Also, such processes generally require the use of filtration procedures which can be time-consuming and expensive. U.S. Pat. No. 2,760,970 describes a process for the preparation of substantially pure metal salts of organic sulfonic acids wherein the inorganic sulfonating acid contaminants are removed as ammonium salt crystals. More specifically, the process involves treating a sulfonated acid mass containing the desirable organic sulfonic acid components and undesirable inorganic sulfonating agent acid contaminants with substantially anhydrous ammonia to neutralize at least substantially all of the inorganic sulfonating agent acid contaminant present, but less than 75% of the organic sulfonic acids to form as a solid phase, the ammonia derivative of said inorganic sulfonating agent contaminants. The solid contaminants are separated from the organic components, and the organic components are thereafter converted to desirable and useful metal salts of the organic components. U.S. Pat. No. 3,720,707 also describes a process for reducing the sulfuric acid content of alkyl aryl sulfonic acids by treating said sulfonic acids with ammonia or an ammonium salt in a quantity to form crystalline ammonium acid sulfate which is separated such as by filtration.
The preparation of useful polyvalent metal sulfonates involves, in general, a double decomposition reaction of an alkali metal or ammonium sulfonate with a particular polyvalent metal compound. When crude sulfonated acid masses are utilized in the double decomposition reaction, many undesirable side reactions occur, and it is often difficult to separate the organic and inorganic salts in the resulting mixture because the polyvalent metal sulfonates disperse the undesirable polyvalent metal sulfates. The result is a hazy solution which cannot be clarified conveniently or is a very viscous solution which must be subjected to additional treatment before it is satisfactory for many uses. Consequently, it is frequently preferable to utilize more expensive organic sulfonic acid mixtures which contain fewer impurities, or it is necessary to purify commercially available organic sulfonic acid mixtures before converting said mixtures to the more useful metal salts. It also has been discovered that neutral and alkaline metal sulfonates exhibiting improved properties can be obtained when the amount of low equivalent weight organic sulfonic acid present in the starting acid mass is reduced. Generally, the lower equivalent weight organic sulfonic acids are polysulfonic acids such as disulfonic and trisulfonic acids. Thus, a procedure which is effective in removing both the inorganic contaminants and the undesirable low equivalent weight organic sulfonic acids from the starting acid mass is desirable.